VulnerabilityPublished 24 Apr 2025Updated 27 Oct 2026

CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies

Affects h11Source GCVE Database

Not known to be exploited · CVSS 9.1 Critical : upgrade to 0.16.0 as a priority.

Data as of 27 Oct 2026, 17:30 UTC

CVSS 3.1 base score

9.1Critical

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 0.16.0

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 24 Apr 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade h11 to 0.16.0.

Affected software

Product	Affected	Fixed in
PyPIh11	Affected <0.16.0	Fixed in 0.16.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade h11 to version 0.16.0 or later, where the parsing of chunked encoding bodies is correctly validated. Alternatively, fix or replace any buggy proxies that incorrectly parse chunked encoding. Since exploitation requires both h11 and a buggy proxy, remediating either component is sufficient to mitigate the risk.

Description

h11 versions prior to 0.16.0 improperly parse the trailing bytes in HTTP/1.1 chunked transfer encoding bodies, accepting malformed chunk terminators. This leniency can cause discrepancies in how h11 and certain buggy proxies interpret HTTP requests, enabling HTTP request smuggling attacks. An attacker could exploit this to bypass security controls or steal session credentials. The issue is fixed in h11 version 0.16.0.

Impact

This vulnerability can enable HTTP request smuggling attacks when a buggy h11-based server is used behind a proxy that also has flawed chunked encoding parsing. Attackers can bypass security controls on the proxy or cause session credential leakage by manipulating request boundaries. The CVSS score of 9.1 reflects critical impact with high confidentiality and integrity consequences but no availability impact.

Summary

The h11 HTTP/1.1 library up to version 0.14.0 does not strictly validate the trailing CRLF sequence in chunked transfer encoding bodies, accepting any two bytes instead. This behavior is harmless alone but can lead to request smuggling when combined with proxies that parse chunked bodies differently, such as reading until a CRLF rather than exactly two bytes. This mismatch allows an attacker to craft requests that are interpreted differently by the proxy and the backend server, potentially bypassing access controls or causing session credential leakage. The vulnerability is identified as CVE-2025-43859 and fixed in h11 version 0.16.0.

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') (opens in a new tab)
Base weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Implementation
Use a web server that employs a strict HTTP parsing procedure, such as Apache.
Implementation
Use only SSL communication.
Implementation
Terminate the client session after each request.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Response Smuggling (opens in a new tab)
HTTP Request Smuggling (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 17:30 UTC

WeaknessesCWE-444

Technical details

References

Community reviews

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